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[1]

Driver choke mod: A small RF choke (around 10 uH) should be connected between R13 and U1 pin 7. Lift the U1 pin 10 side of R13 and put one end of the new coil in the opened hole. Solder the free ends of the coil and resistor together. This mod improves the drive to the output transistor and keeps stray RF out of the power supply circuit. Keep the lead lengths as short as possible.

[2]

Final amp choke and resistor mod: Replace R9 (270 ohms) with a series combination of a small RF choke (around 10 uH) and a 200 ohm 1/4 watt resistor. This mod in combination with the driver choke mod brings the output of the transmitter up to about 10 milliwatts and once again keeps RF out of the power supply. The stock FM-10a puts out around 5 milliwatts of power. Don't do this mod without also doing the lowpass filter mod, you wouldn't want harmonics showing up on the police bands, would you. Of course, in the USA this power level is too high anyway so you should only use this circuit to warm up a shielded dummy load. The resistor value is set for maximum power with a 12 volt supply on the FM-10a. Smaller resistor values will gain you nothing and would possibly overheat and destroy the transistor Q1. Short lead lengths are a must.

[3]

Lowpass filter mod: *IMPORTANT* Put the following circuit between C13 and the output jack, it eliminates harmonics that may interfere with other frequencies and cause unwanted attention to be drawn to you. It is best to build the filter into its own metal box and run coax cable to it via BNC connectors. The capacitors may be silver mica or disks. The coils L1, L2, and (L3) can be made by wrapping four turns of tinned bare #20 gauge wire around a 7/16" diameter form such as a Sharpie pen. Form the coil so that the wires come off of the same side of the coil and at right angles to the coil. Wind the coil tight and pull it apart so that there is about 1/32" spacing between the turns. Build this circuit on a small piece of copper clad circuit board and make a wide solder connection between the filter board and the main circuit board for a good RF ground connection. I soldered one filter board to the back side of the output jack with good results. Some adjustment of the spacing may be necessary, the best way is to connect the output to a cheap CB SWR meter with a 50 ohm dummy load on the output, spread or squeeze the turns of the two coils to the point just before the power level drops on the meter. Beware that a CB SWR meter may not work well at 100 Mhz, mine does. If you have access to a network analyzer or a spectrum analyzer or a scope and sweep generator, they can make tuning the filter easier. Put the filter into a metal box with BNC or other coax connectors on the input and output connections coming out of opposite ends of the box. A mica compression trimmer may be substituted for the input 33pf capacitor, tune the capacitor to the point where the output level just starts to fall off.

 

[4]

VFO capacitor mod: The capacitor supplied with the kit for C16 is not very stable for use in a VFO circuit. Replace C16 with an equivalent value NPO ceramic capacitor (zero temperature coefficient). This will improve stability considerably. NPO capacitors are available from Digi-Key and Newark in the USA. If you can find one, a ceramic Johanson coaxial trimmer capacitor will work nicely as a replacement for C16.

[5]

VFO voltage regulator mod: An inexpensive adjustable voltage regulator IC can be substituted for the low quality temperature sensitive diode string regulator in the FM-10a. This mod along with the VFO capacitor mod will improve the frequency stability of the FM-10a greatly. Installation instructions follow: First Remove the BA1404 IC. Remove one or all 3 of the 1N914 diodes, D1 thru D3. Remove resistor R8. Install the input side of an LM317L regulator in the hole where the S1 side of R8 was. Install the output side of the LM317L in the hole where the D1 side of R8 was. Take two 220 ohm 1/4 watt resistors and twist them in series. Connect one end of the two resistors to the circuit board ground plane. Connect the other end of the two resistors to the voltage regulator output pin. Connect the voltage regulator reference input pin to the middle of the two resistors. Connect an 0.1uF disk capacitor between the circuit board ground and the input pin of the LM317L. With the BA1404 IC (U1) still removed, test the operation of the regulator. Apply 9 to 12 volts to the FM10a, turn the power switch on, and measure the output pin of the regulator relative to ground. If the regulator is working then it's output voltage is around 2.5 volts. The output voltage should be no more than 3.2 volts, more will damage the IC. If you can very the input voltage, do so, the output voltage should remain stable. When everything checks out ok, plug the BA1404 back into its socket.
 
 
 

Parts list:
2	10 uH RF chokies, the value is not too critical but pick small parts.
1	200 Ohm 1/4 watt resistor.
2	33 pf silver mica capacitors.
1	56 pf silver mica capacitor.
2	0.125 uH coil: 4 turns of #20 wire on a 7/16" diameter form spaced 1/32".
1	33* pf NPO ceramic capacitor *use the same value as the old C16 cap.
1	LM317L TO-92 case adjustable voltage regulator.
2	220 Ohm 1/4 watt resistors.
1	0.1 uf capacitor, disk or monobloc type.

 
All of the parts can be obtained from Digi-Key, 1-800-DIGI-KEY or other electronics distributors.

Remember, as with all RF circuits, the FM10 will work much better if it is securely mounted in a METAL box and wire leads are kept to a minimum. Don't waste your time with plastic boxes. Many thanks go to Mycal and his FM10 faq for inspiring many of these mods.

• Addendum - All BA1404 based kits that I have seen so far have a broad spectrum output near the desired frequency of operation, this can cause interference problems. A simple fix to this is to put a tuned circuit right on the output of the BA1404 (pin 7). A 1nf DC blocking capacitor in series with a parallel resonant tuned circuit cleans the signal right up. For the tuned circuit, I used a 5-45 pf compression trimmer (type 403) across a 3 turn 3/8" diameter #20 AWG air coil. The coil should be put right on the trimmer with very small (< 1/4") leads. Keep all leads short and solder one side of the LC to a ground that is close to the BA1404 ground. Connect the 1nf capacitor between the top of the LC circuit and BA1404 pin 7. Adjust the circuit for the highest output power, it may be necessary to compress or expand the coil a bit to achieve a real peak. If you have access to a good scope or spectrum analyzer, these can help in the alignment process. Tuning should be fairly broad for this circuit. A very useful test is to operate an fm receiver near the transmitter antenna, try this before and after the mod, you should notice a drop or complete elimination of your signal everywhere in the band but where it is tuned to. To boost the gain in the following amplifier transistor, it may help if you tap the amp's input capacitor at about half way up the coil in the new LC circuit, this will give a better impedance match.

Also: Most of the BA1404 kits I have built have had a real problem with RF feedback coming in from the power and audio lines, especially in the vicinity of medium power RF. I have found the following things to help out. First, enclose the BA1404 kit in a metal box. Second, use a full metal BNC connector (not an insulated ground type) and install that very close to the RF output on the circuit board. Run wires from the ground and hot leads of the BNC connector right to the board. Third, use metal spacers to mount the board in the box. Be sure you don't short the circuit board with the spacers. Fourth, build low-pass filters on the audio inputs and DC power input. The Audio low pass filter consists of two identical pi filters with 10uH coils and 100pF caps, the caps go to ground on one side and to either end of the coil on the other side, one side of the coil goes to the FM10 input and the other to a coax wire that runs to RCA jacks mounted on the metal box. Ground the coax to the circuit board on one end and to the metal box on the other side. The DC power should also connect to the board via a similar dual-grounded coax line. A similar pi filter should go on this line but the components are 0.01uF for the caps and 100uH or so for the coil. This filter should go right on the DC input connector on the metal box, I have used a banana plug for this. Keep all filter leads very short and ground the power filter to a lug on the metal box. All of this might seem like a lot of trouble but it will save you time in the long run. I also recommend installing a temperature controller circuit in the same box to keep the oscillator at a constant temperature, this greatly reduces drifting.